phy: renesas: rcar-gen3-usb2: add is_otg_channel to use "role" sysfs

Even if a board doesn't have otg pins connection, this hardware can
change the role by a register setting. So, this patch adds
"is_otg_channel" for it.

Signed-off-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Reviewed-by: Simon Horman <horms+renesas@verge.net.au>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>